1. Field of the Invention
The present invention relates generally to code generation in a data communications system, and in particular, to an apparatus for generating complementary turbo codes, considering the characteristics of turbo codes in a packet communications system or a general communications system that employs a retransmission scheme.
2. Description of the Related Art
In general, a system using a retransmission scheme (e.g., HARQ: Hybrid Automatic Repeat Request) performs soft combining to improve transmission throughput. The soft combining techniques are divided into packet diversity combining and packet code combining. These two combining schemes are usually called soft packet combining. Although the packet diversity combining scheme is sub-optimal in performance relative to the packet code combining scheme, it is favorable due to easy implementation when performance loss is low.
A packet transmission system uses the packet code combining scheme to improve transmission throughput. A transmitter transmits a code with a different code rate at each packet transmission. If an error is detected from the received packet, a receiver requests a retransmission and performs soft combining between the original packet and a retransmitted packet. The retransmitted packet may have a different code from the previous packet. The packet code combining scheme is a process of combining received N packets with a code rate R to a code with an effective code rate of R/N prior to decoding, to thereby obtain a coding gain.
With regard to the packet diversity combining scheme, on the other hand, the transmitter transmits the same code with a code rate R at each packet transmission. If an error is detected from the received packet, the receiver requests a retransmission and performs soft combining between the original packet and the retransmitted packet. The retransmitted packet has an identical code to that in the previous packet. In this sense, the packet diversity combining scheme can be considered the received symbol energy averaging on a random channel. The packet diversity combining scheme reduces noise power by averaging the soft outputs of the received input symbols and achieves such a diversity gain as offered by a multi-path channel because the same code is repeatedly transmitted on a fading channel. However, the packet diversity combining scheme does not provide such an additional coding gain as obtained according to a code structure in the packet code combining scheme.
A turbo encoder for generating the turbo code will be described herein below. In the case of a turbo encoder with R=1/5, the turbo encoder generates information symbols X, first parity symbols Y0, Y0′ and second parity symbols Y1, Y1′ by encoding input information symbols. The turbo encoder is comprised of two constituent encoders and one interleaver. The first parity symbols Y0 and Y0′ are output from a first constituent encoder by encoding the input information symbols and the second parity symbols Y1 and Y1′ from a second constituent encoder by encoding the information symbols interleaved through the interleaver. In detail, the Y0 is a row of first parity symbols generated from a first constituent encoder, and the Y0′ is a row of second parity symbols generated from the first constituent encoder.
Due to implementation simplicity, most packet communication systems have used the packet diversity combining scheme, which is under study for application to the synchronous IS-2000 system and the asynchronous UMTS system. The reason is that the existing packet communication systems have used convolutional codes and even packet code combining does not offer a great gain when convolutional codes with a low data rate are used. If a system with R=1/3 supports retransmission, there is not a wide difference in performance between the packet code combining scheme and the packet diversity combining scheme. Thus, the packet diversity combining scheme is selected considering implementation complexity. However, use of turbo codes as forward error correction codes (FEC) requires a different packet combining mechanism because the turbo codes are designed as error correction codes to have performance characteristics very close to the “Shannon Channel Capacity Limit” and their performance varies obviously with the coding rates unlike convolutional codes. Therefore, it can be concluded that packet code combining is desirable for a packet communication system using turbo codes in a retransmission scheme to achieve the goal of optimum performance.